The treasure chest decoration is an item that is obtained from prawn balls with a chance of 3%. It can be added to an Aquarium. Once placed, it can be opened once a week to receive an elite clue scroll. The chest resets on Wednesdays.[1]

If the player already has an elite clue scroll when attempting to open the chest the following message is displayed in the chatbox:You already have a clue scroll. Solve that and come back to claim a new one.